diff --git a/core/src/vfs.sld b/core/src/vfs.sld
index 55ddba3..25eab4a 100644
--- a/core/src/vfs.sld
+++ b/core/src/vfs.sld
@@ -100,13 +100,13 @@
       (if (string=? subdir "")
        vfs
        (make-vfs
-        (mapping-map
-         (lambda (key value)
+        (mapping-fold
+         (lambda (key value acc)
            (cond
-             ((string=? (car key) subdir) (values (cons "" (cdr key)) value))
-             ((string-prefix? subdirprefix (car key)) (values (cons (string-copy (car key) subdirprefixlength) (cdr key)) value))
-             (else (values))))
-         (make-default-comparator)
+             ((string=? (car key) subdir) (mapping-set! acc (cons "" (cdr key)) value))
+             ((string-prefix? subdirprefix (car key)) (mapping-set! acc (cons (string-copy (car key) subdirprefixlength) (cdr key)) value))
+             (else acc)))
+         (mapping (make-default-comparator))
          (vfs-contents vfs)))))
 
     ;; Generates a full VFS structure from an on-disk directory.